THE EFFECT OF SITE DISTRIBUTION ON DESORPTION-KINETICS - CARBON-MONOXIDE FROM NI(100)

The effect of binding site distribution of CO on its desorption kinetics from Ni(100) was investigated using temperature program desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). Desorption of CO was studied from either surfaces presaturated at 100 K and annealed to 250 K (theta(co) = 0.59 ML), 342 K (theta(co) = 0.49 ML), and 414 K (theta(co) = 0.21 ML) or by dosing to similar coverages without anneal at 100 K. The TPD spectra indicate that the initial desorption kinetics are significantly different for the two methods of preparation. Desorption kinetic parameters, E(d) and v(d), obtained using the threshold temperature program desorption (TTPD) analysis without anneal were 114 +/- 2 kJ/mol and 8.1 x 10(15) s(-1) at 0.18 ML and 32 +/- 2 kJ/mol and 10(7) s(-1) at 0.50 ML. At both of these coverages CO is initially distributed nearly equally between atop and bridged sites. Annealing the adlayer led to the dominance of atop sites in the adsorbate population, yielding less coverage-sensitive desorption kinetics. At theta(co) = 0.21 ML, E(d) and v(d) were 128 +/- 4 kJ/mol and 3.9 X 10(16) s(-1), and at 0.50 ML they were 92 +/- 3 kJ/mol and 1.3 x 10(14) s(-1).